Methods for scanning an examination object with a CT system are generally known, with use being made, for example, of circular scans, sequential circular scans with feed or spiral scans. During these scans absorption data on the examination object is captured from different capture angles using at least one x-ray source and at least one opposed detector, and the absorption data or projections collected in this way are allocated to sectional views through the examination object by way of corresponding reconstruction methods.
For reconstruction of computed-tomography images from x-ray CT datasets of a computed tomography device (CT device), i.e. from the projections captured, use is made nowadays of what is known as a Filtered Back Projection (FBP) method as a standard method. Following the data capture, what is known as a “rebinning” step is carried out, in which the data generated with the fan-shaped beam extending from the source is rearranged so that it is present in a form as if the detector was being hit by x-rays approaching the detector in parallel. The data is then transformed into the frequency range. Filtering takes place in the frequency range, and then the filtered data is transformed back. A back projection onto the individual voxels within the volume of interest is then performed using the thus rearranged and filtered data. In traditional FBP methods the image definition is linked to the image noise. The higher the definition achieved, the higher the image noise too, and vice versa.
While the CT-measured data is being captured, the examination object, generally a patient, is exposed to a dose of x-ray radiation. Since this radiation is generally not harmless for the examination object an effort is made to get by with the least possible radiation exposure. The dose used is however directly linked to the image noise in the image data reconstructed from the CT-measured data: a reduction in the dose leads to an increase in the noise. To make maximum use of a particular radiation dose it is therefore worthwhile to use image reconstruction methods which efficiently reduce the noise in CT images.